1. Field of the Invention
This invention relates generally to a nondestructive test method for flexible composite articles, more particularly to a method of testing cord penetration in reinforced rubber articles, and specifically to a test for cord penetration in polyurethane power transmission belts.
2. Description of the Prior Art
A number of nondestructive test methods are known for structures of rigid composites and metals. Examples of such methods include use of ultrasonics, eddy currents, and acoustics. Composite or metal structures can often be tested for internal defects by tapping on them and listening for differences in the sound of the tap. The so-called coin tap test or simply, “tap testing” has been applied to various types of structures of rigid materials, including honeycomb composites, aircraft, space structures, buildings, etc. Tap testing may be used to detect delamination, crushed or missing internal support structures, debonding, etc. in such rigid structures. Tap testing systems range from hand-tapping by a person with a trained ear, to automated tappers with electronic analysis of the sound or impulse.
U.S. Pat. No. 6,327,921 to Hsu et al. describes a nondestructive, tap-testing, inspection system which provides an image-based display of a plurality of structural stiffness measures, each stiffness measure calculated from a measure of impact duration for a “tap” useful for aircraft structures and composite and metal honeycomb structures. Other examples of tap-testing systems are disclosed in U.S. Pat. No. 5,048,320 to Mitsuhashi et al. and in U.S. Pat. No. 4,519,245 to Evans.
Flexible composite articles, such as tires, power transmission belts and hose, may have internal defects such as voids, delaminations, and the like. U.S. Pat. No. 5,891,561 to Kinoshita et al. describes the use of load carrying cords of aramid or other fibers in rubber power transmission belts. According to that patent, when treating the cords with resorcinol-formaldehyde-latex (“RFL”) treatments, if the percentage of voids after treatment is greater than 1.5%, the voids become relatively large, and the cohesion between filaments in the cords decreases and a fraying problem becomes significant. The method proposed to determine the percentage of voids is to enlarge a cross section of the cord with an electron microscope to allow precise area measurements.
U.S. Pat. No. 5,807,194 to Knutson et al. describes the history of cord development for belts with cast polyurethane bodies, emphasizing the importance of cord penetration by the various treatments or belt materials used. For the carbon cords of primary interest, Knutson et al. propose a destructive test for measuring cord penetration. According to that patent, the amount of belt material that a cord picks up during casting can be measured by weighing a length of greige cord and comparing it to a cord that is dislodged from a finished belt and measuring the weight difference. In this manner, cord pick up of belt material in mg per mm3 of cord volume can be determined for each millimeter of cord length. In practice this test may be confirmed by studying magnified cross sections of the cord.
Both cord weighing and cross-section analyses are destructive tests.